VP of Marketing Suzanne Lowry wrote on T-Mobile's Insights & Issues Blog this week that the carrier starting Sept. 12 -- the same day Apple is expected to introduce the iPhone 5 -- will step up efforts "to attract customers with unlocked iPhones to T-Mobile."

T-Mobile emphasizes that its unlimited nationwide data plan can save customers $50 a month versus going with AT&T (no mention of Verizon or other carriers, except to say that Verizon and Sprint iPhones won't work on T-Mobile's network), and boasts that iPhone users on its 4G HSPA+ network typically get much faster speeds than on AT&T's network. The catch is that T-Mobile is still rolling out its 4G network, currently available mainly in big cities such as Las Vegas, New York City, Seattle and Washington, D.C.

The fourth-largest U.S. carrier says it is prepping to handle an influx of unlocked iPhones on its network by having 4S demo units on hand at all retail locations and developing T-Mobile billing, voicemail and other apps for iOS. T-Mobile claims to serve more than 1 million iPhone users on its network already.

T-Mobile's pitch comes just as Apple is planning to announce an iPhone 5 model most industry watchers expect will run on 4G LTE networks from AT&T, Sprint and Verizon, as well as other carriers around the world, out of the gate. It also comes at a time when the used iPhone market will be flush with iPhone 4 and 4S models as Apple fans look to upgrade (the question is whether many of these will actually be unlocked). Unlocked iPhones typically cost more than the locked versions linked to certain carrier networks, though AT&T for one recently has begun to make it simpler for customers to unlock their phones once their contracts are up.

Bob Brown is a news editor for Network World, blogs about network research, and works most closely with our staff's wireless/mobile reporters. Follow him on Twitter at Alphadoggs and connect via email at bbrown@nww.com